A Reusable Thermochromic Phantom for Testing High Intensity Focused Ultrasound Technologies

A high magnetic resonance imaging (MRI) contrast agar/silica-based phantom for evaluating focused ultrasound (FUS) protocols

PURPOSE

Thermal ablation therapies require tissue mimicking phantoms for evaluating novel systems. Herein, an agar phantom exhibiting high magnetic resonance imaging (MRI) contrast to noise ratio (CNR) was developed for testing focused ultrasound (FUS) protocols.

METHODS

Four agar based phantoms (6 % w/v) were fabricated with varied silica concentrations (0, 2, 4, or 6 % w/v) and subjected to FUS inside a 3 T MRI. T2-Weighted Fast Spin Echo (T2-W FSE) images were acquired after sonications to assess the effect of varied silica on CNR of inflicted lesions. The highest CNR phantom was sonicated and its proton resonance frequency (PRF) coefficient, thermal dose denaturation threshold and ability to sustain good lesion CNR 0-44 min post exposures were assessed.

RESULTS

T2-W median lesion CNR between 1.5-453.5 was observed, exponentially increasing with increased silica concentration. High CNR was achieved with 4 % w/v silica, with the PRF coefficient of the phantom calculated at -0.00954 ppm/°C. The thermal dose denaturation threshold was revealed at 2 × 106 CEM43°C by comparing thermal dose maps with T2-W FSE lesion hyperenhancement. Progressive lesion CNR loss was observed, with CNR lost 28 min after sonications.

CONCLUSIONS

The proposed phantom possesses excellent T2-W contrast of inflicted lesions while exhibiting a tissue like PRF coefficient and can thus constitute an inexpensive reusable tool for validating FUS systems and protocols.

Silicone-based materials with tailored MR relaxation characteristics for use in reduced coil visibility and in tissue-mimicking phantom design

BACKGROUND

The development of materials with tailored signal intensity in MR imaging is critically important both for the reduction of signal from non-tissue hardware, as well as for the construction of tissue-mimicking phantoms. Silicone-based phantoms are becoming more popular due to their structural stability, stretchability, longer shelf life, and ease of handling, as well as for their application in dynamic imaging of physiology in motion. Moreover, silicone can be also used for the design of stretchable receive radio-frequency (RF) coils.

PURPOSE

Fabrication of materials with tailored signal intensity for MRI requires knowledge of precise T1 and T2 relaxation times of the materials used. In order to increase the range of possible relaxation times, silicone materials can be doped with gadolinium (Gd). In this work, we aim to systematically evaluate relaxation properties of Gd-doped silicone material at a broad range of Gd concentrations and at three clinically relevant magnetic field strengths (1.5 T, 3 T, and 7 T). We apply the findings for rendering silicone substrates of stretchable receive RF coils less visible in MRI. Moreover, we demonstrate early stage proof-of-concept applicability in tissue-mimicking phantom development.

MATERIALS AND METHODS

Ten samples of pure and Gd-doped Ecoflex silicone polymer samples were prepared with various Gd volume ratios ranging from 1:5000 to 1:10, and studied using 1.5 T and 3 T clinical and 7 T preclinical scanners. T1 and T2 relaxation times of each sample were derived by fitting the data to Bloch signal intensity equations. A receive coil made from Gd-doped Ecoflex silicone polymer was fabricated and evaluated in vitro at 3 T.

RESULTS

With the addition of a Gd-based contrast agent, it is possible to significantly change T2 relaxation times of Ecoflex silicone polymer (from 213 ms to 20 ms at 1.5 T; from 135 ms to 17 ms at 3 T; and from 111.4 ms to 17.2 ms at 7 T). T1 relaxation time is less affected by the introduction of the contrast agent (changes from 608 ms to 579 ms; from 802.5 ms to 713 ms at 3 T; from 1276 ms to 979 ms at 7 T). First results also indicate that liver, pancreas, and white matter tissues can potentially be closely mimicked using this phantom preparation technique. Gd-doping reduces the appearance of the silicone-based coil substrate during the MR scan by up to 81%.

CONCLUSIONS

Gd-based contrast agents can be effectively used to create Ecoflex silicone polymer-based phantoms with tailored T2 relaxation properties. The relative low cost, ease of preparation, stretchability, mechanical stability, and long shelf life of Ecoflex silicone polymer all make it a good candidate for "MR invisible" coil development and bears promise for tissue-mimicking phantom development applicability.